When is a 3v battery considered (close)to empty

Obviously ,that detail is needed for a low battery indicator LED circuit ,(1 led would pulse at a very low freq to show the low battery level and with a very low duty cycle) .Using a lm2903 ,as i think it's capable of operating at such a low power level of 1.5/2 Vcc and a CR2032 coin cell 3V Lithium battery ?

I want to incorporate this into an cmvs ,which has already had the battery mod,so doing an LED indicator to show when it's time to change the cell ,will come in handy.

Any help or tips ,please give them.

 

I pulled a 3v CR2032 out of a Saturn yesterday that hadn't had the tab pulled out so it could be used for the RTC in 15 years. It measured at 3.7v.

 

Get one of these, take your multimeter out, learn from it.

http://cgi.ebay.co.uk/Battery-Check...Accessories_SpareParts_SM&hash=item5d2e6cdf20

Of course, it's a bit different if it's under load, and it'll depend on the application - so ultimately, only you can answer your question.

The tester will give you an idea, but the answer is that the battery is "dead" when your device won't run properly any more - and that'll vary from device to device (as an example, a remote control or a GBA may continue to work on batteries that would have stopped working in a flash gun, which requires more current draw).

 

As retro says it does depend heavily on what the battery is being used for as, different things require different power draws. Depending on the battery chemistry will also determine battery life.

CR lithium batteries tend to show a voltage of around 3V to 2.9V under a few ma load but the voltage will drop quickly once it reachs 2.8V, so effectively you will have to build a circuit that does not leach too much power to measure a 0.1V drop. Good luck.

For information (for 1.5V AA batteries)

Zinc Carbon batteries are considered dead at 0.9V, as they drop to 1.2V very quickly before discharging slowly.

Akaline batteries is 1.1V, and they tend to discharge slowly to this voltage before dropping quickly.

NiMH rechargables are 1.2V to start with and they tend to hold voltage quite well before dropping very sharply once then get to 1.1V.

Lithium AA batteries act like the CR batteries as the offer a good steady 1.4V voltage for most of the life of the battery before dropping very quickly.

If you ran a gameboy on these batteries you would notice that the low battery indicator would lie about the life of NiMH (as it would say they were dead for a long time before the console finally died) and Lithium (as it would say the battery is fine before fading away and the console turning off).

 

Using a CR2032 with a 5mm LED (that's power hungry) you'd get about 12 - 24 continuious hours out of it before the battery slowly drained, put out less and less power and eventually could be considered useless.